Experimental results and plans of Versatile Experiment Spherical Torus (VEST)
Ohmic operation of VEST (Versatile Experiment Spherical Torus) has been successfully carried out, generating plasma currents of up to 70 kA with pulse duration of ~15 ms at the toroidal magnetic field of 0.1T on axis.
The elongation and edge safety factor of the low aspect ratio plasmas (R=0.4m and a=0.3m) are estimated to be 1.6 and 3.7 respectively from equilibrium reconstruction based on magnetic diagnostics.
Efficient ECH (Electron Cyclotron Heating) assisted plasma start-up scheme utilizing the trapped particle configuration (TPC), a mirror like magnetic field configuration, is developed as a new approach to improve the conventional start-up scenario based on field null formation with long connection length.
TPC significantly enhances the pre-ionization with the enhanced particle confinement due to its mirror effect, and stable decay index in this configuration also enables the prompt plasma current initiation.
Consequently, TPC demonstrates robust tokamak start-up with lower loop voltage with lower volt-second consumption as well as wider operation range in terms of the ECH pre-ionization power and the hydrogen filling pressure.
Direct mode conversion of X-mode to EBW (Electron Bernstein Wave) from low field side is successfully utilized to enhance the ECH pre-ionization by generating over-dense plasmas beyond L-cutoff density.
To study advance tokamak regime with high beta and high bootstrap operation in VEST, high power neutral beam heating is under preparation along with profile diagnostics. Various wave heating and current drive methods such as EBW and low hybrid fast wave are under preparation for current density profile control.
In the meantime, ohmic plasmas will be improved to provide proper target plasmas for high power heating, especially for neutral beam injection by increasing plasma density with higher plasma current at longer pulse length and higher magnetic field.